We demonstrated the existence of immunoreactivity to 2 amphibian peptides, bombesin and physalaemin, in extracts of a human lung small-cell carcinoma which was propagated in nude (athymic) mice. These immunoreactive peptides were characterized by reverse-phase HPLC coupled to various chemoselective and enzymic modifications. In the presence of a hydrophilic ion-pairing reagent, the rention time of both peptides were reduced by about 2.5 min relative to pH 4.2. At pH 7.0, only bombesin was more strongly retained to the hydrophobic column due to the loss of charge on His. Pyro-glutamate aminopeptidase also produced a bombesin molecule with a higher retention time whereas physalaemin immunoreactivity was lost, confirming the NH2-terminal specificity of the antibody. Trypsin cleaved both tumor peptides producing fragments similar to the standards. Selective oxidation by oxone confirmed the presence of a sulfur-containing residue (Met).